What type of aquifer is characterized by layers of non-porous materials above and below it?

A confined aquifer is characterized by being situated between layers of non-porous materials, such as clay or rock, both above and below it. This stratification creates pressure within the aquifer, which can lead to water being stored under pressure and sometimes results in artesian conditions. When a well is drilled into a confined aquifer, water can rise to the surface without being pumped if the pressure is high enough, demonstrating the unique qualities of this type of aquifer.

In contrast, unconfined aquifers do not have a layer of impermeable rock or clay atop them, allowing water to percolate down from the surface more freely. Perched aquifers, on the other hand, form above a regional aquifer when a localized layer of impermeable material allows water to accumulate. The term "groundwater aquifer" is a more general term that doesn't specifically refer to the characteristics of confinement.

Thus, the defining feature of a confined aquifer is its encapsulation between non-porous materials, making it distinct from other types of aquifers.